The number of British beaches recommended for excellent water quality has fallen after the wettest summer on record last year.

The Marine Conservation Society (MCS), which produces an annual guide to Britain's best beaches, said the number of bathing beaches reaching the highest standards had dropped by 10% - the biggest year on year fall in the guide's 21-year history. Beaches that failed to reach minimum legal standards more than tripled.

Water samples, which are tested by the Environment Agency each year between May and September, showed a decline in water quality, which is being blamed on the exceptionally heavy rainfall in June and July last year.

Thomas Bell, coastal pollution officer for the MCS, said: "When you get a lot of water from storms you get storm-related pollution, which is the runoff from agriculture, animal and urban waste and petrochemicals which run into rivers and streams. Pollution also comes from combined sewer overflows, which are long pipes, which also shoot flood water. Last summer it happened a lot and it's why we've had a record drop in the number of beaches we've awarded."

The MCS guide is based on European mandatory water standards and treatment of continuous sewage discharges, with recommended status given to beaches that achieve an excellent rating in both categories. The organisation also recognises three other levels: guideline, based solely on water quality, basic pass and fail.

This year's survey of 778 beaches recommends 434 beaches, 370 of which were also recommended last year. Ninety-one beaches fell from recommended status, and 10 of these are now listed as a fail. Beaches that have dropped to last place after being recommended last year include Summerleaze beach in Bude, Cornwall, and Runswick Bay in North Yorkshire. The trend over the past 10 years has been to recommend more beaches.

The MCS conceded that some beaches may now be clean. Bell said: "The water quality will probably have changed a lot since last year - it can change on a daily, even hourly, basis. Given that we have this imperfect testing, the best thing to do is to look at the bigger picture, to see if over the past three years a beach has been consistently recommended.

"If we get another summer like last year, I would pick beaches that have done well in this year's guide. If they've done well, they've borne the brunt of the storms and are probably storm-proof."

The alternative is to adopt a system of continual monitoring, being used on 10 beaches in Scotland, where swimmers can look at electronic signs which give up-to-date information on water quality.

The system was put in place with funding from the Scottish executive after water standards, which were generally high, were found to be at risk of not meeting European standards after wet weather. The daily forecast is also available online and via phone and SMS services.

A revised European bathing water directive was adopted in England in March this year. It calls for tighter minimum water quality standards by 2015 and broader regulations for informing the public.

The MCS warned that specific measures for dealing with pollution risks from bad weather - expanding sewer systems and better management of combined sewer overflows - are also needed.

Blue Flag, an international award scheme covering 36 countries, will announce its annual list of cleanest beaches next month.